Appears to be a certificate commemorating a line-crossing ceremony, the first time a sailor crosses the Equator. (You can see on the certificate that the latitude of the Equator is pre-printed, while the longitude has been filled in by hand). Many navies celebrate one --- particulars vary, but basically the new sailors have to go through some humiliting but fairly harmless ritual to be "inducted" into the court of Neptune. The officers and/or most experienced sailors dress up as Neptune's court, it's basicaly an excuse for the ship to party a bit during a long voyage. U.S.S. Chester seems to have been a battle cruiser operating mostly in the Pacific. The longitude puts it off the coast of Ecuador.

There are similar rituals for the first crossing of the international date line. Sailors are a superstitious bunch.

In case you're short on answers, you might have better luck asking this in /r/whatisthisthing/

Anyway, good luck. :)